This is a continuation-in-part of our earlier application Ser. No. 501,743 filed Aug. 29, 1974, now U.S. Pat. No. 3,908,862 and entitled Ratio Controlled Mixing of Liquids and owned by a common assignee which is expressly incorporated herein (hereinafter referred to as the parent or parent case).
This invention relates to an improvement in apparatus for the ratio controlled mixing of liquids. One aspect of the invention relates to such apparatus having electrical automatic control loops for each of at least two liquids arranged in a master-slave relationship between the control loops. An aspect of the invention is automatic control apparatus using electrical feedback automatic control systems where the master system has a programmable velocity profile reference and provides from its own feedback loop a ratioed velocity signal to each slave control system that is representative of actual master velocity. The invention has particular application to the feeding and mixing of reagents that include polyol and isocyanate in the liquid injection molding of articles made of polyurethane composition in connection with which the present invention will now be described by way of example but not limitation.
Liquid feeding mixers having means to adjust the feed ratio of one or more liquids to another are known, as is taught in a large number of previously issued U.S. and foreign patents, of which U.S. Pat. Nos. 3,386,623 and 3,642,175 are generally representative. It is also known to feed such liquids using plunger or piston type metering pumps.
It is common practice in the prior art to feed the reagents for making polyurethane sequentially or one at a time into a mixing chamber. This, however, generates a number of difficulties such as requiring longer cycle time for molding operations and also requiring a longer shelf time or pot-life to avoid having the reagents set up in the mixing chamber. One of the features and advantages of the instant invention is to reduce cycle time by allowing the use of fast-reacting chemicals whereupon the cycle time can be substantially reduced.
More specifically, the invention comprehends an apparatus for feeding liquids in a preselected ratio from at least two separate sources, each having automatic control system arranged in a velocity coupled master and slave relationship to effectuate said ratio which includes: first and second positive displacement liquid feeding means (such as a plunger type metering pump or an expandable chamber device); first and second means for measuring the rate at which, respectively, each of said first and second liquid feeding means dispenses its respective liquid (such as an electric tachometer); first and second velocity feedback control system means, for automatically controlling said first and second liquid feeding means, respectively, by feeding back signals from, respectively, said first and second rate measuring means to a control summing junction in the respective control systems velocity ratio control means for receiving an input signal from said first rate measuring means and in response thereto producing as an output signal a ratioed master velocity signal having a preselected ratio to said input signal; velocity signal summing means for receiving and summing said ratioed master velocity signal and said slave velocity signal from said second feedback control system means to produce a feedback error signal; and means to conduct said feedback error signal to the aforesaid control summing junction in said second feedback control system means, whereby said slave velocity signal is summed twice to produce a slave control signal for controlling the velocity or rate at which said second or slave liquid feed means is operated.